The present invention relates to a color cathode ray tube, particularly, an in-line type color cathode ray tube equipped with an in-line type electron gun structure and capable of improving the convergence characteristics of a plurality of electron beams emitted from the in-line type electron gun structure.
In general, an in-line type color cathode ray tube comprises an envelope having a panel 1 and a funnel 2 connected to the panel 1, as shown in FIGS. 1 and 2. A phosphor screen 3 emitting red (R), green (G) and blue (B) lights is arranged inside the panel 1. Also, a shadow mask 4 is arranged close to the phosphor screen 3.
The funnel 2 comprises a neck 5 in which are arranged three electron guns forming an in-line type electron gun structure. These electron guns, which emit three electron beams, are arranged to form a row on a horizontal plane, i.e., in a direction of X-axis.
Further, a deflection device 6 is mounted to the outer circumference of a region extending from the funnel 2 to the neck 5. A two-pole magnet 7 having a set of an N-pole and an S-pole arranged to face each other is mounted in a rear end portion of the deflection device 6. The two-pole magnet 7 serves to control the landing of the electron beams.
A convergence magnet 8 is arranged outside the neck 5. The convergence magnet 8 comprises a pair of ring-like magnet plates 11 consisting of two sets of an N-pole and an S-pole arranged to face each other, totaling four poles, and serving to generate a static magnetic field and a pair of ring-like magnet plates 10 consisting of three sets of an N-pole and an S-pole arranged to face each other, totaling six poles, and serving to generate a static magnetic field.
The two-pole magnet 7 and the convergence magnet 8 collectively serve to permit the three electron beams emitted from the electron gun structure, i.e., central beam for green light emission, and two side beams for red and blue light emission, which are aligned to form a single row, to be landed in the center of the phosphor screen 3 so as to achieve a sufficiently high color purity and convergence. These three electron beams are deflected by the deflection device 6 and scanned so as to reproduce a color picture image on the phosphor screen 3.
In the in-line type color cathode ray tube of the construction outlined above, the electron beams are likely to be affected by an external magnetic field such as geomagnetism. Also, the conditions of the external magnetic field are dependent on the direction in which the color cathode ray tube is disposed because it is possible for the color cathode ray tube to be disposed in a direction differing from the direction in which the convergence of the electron beams is adjusted and on the geometrical location of the color cathode ray tube because geomagnetism differs depending on the geometrical location. Such being the situation, it is possible for the red image and blue image displayed on the phosphor screen as a result of excitation with the side beams to be relatively deviated in the vertical direction. The reasons for the generation of the particular phenomenon are considered to be as follows.
Specifically, in the color cathode ray tube disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 7-250335, an electron gun structure is arranged within the neck. In the electron gun structure in this prior art, the cathode which generates thermoelectrons upon when heated by a heater is formed of a material having a low thermal expansion coefficient and acting as a magnetic body. Therefore, if the external static magnetic field generated by, for example, geomagnetism crosses the tube axis in the neck portion, i.e., Z-axis, the external magnetic field is converged toward the cathode, which is a magnetic body, with the result that forces opposite to each other in direction are exerted on the side beams of the aligned three electron beams.
In other words, the external magnetic field causes the side beams to receive forces opposite to each other in the horizontal component, i.e., X-axis component. For example, where an external magnetic field in a positive direction of the X-axis exerts on the electron beam for red emission, force in a negative direction of the Y-axis (vertical direction) is applied to the electron beam so as to cause the electron beam for red emission to be shifted in the negative direction of the Y-axis. On the other hand, an external magnetic field in a negative direction of the X-axis is exerted on the electron beam for blue emission, with the result that force in a positive direction of the Y-axis is applied to the electron beam for blue emission so as to cause the electron beam to be shifted in the positive direction of the Y-axis. It follows that the red image and the blue image displayed on the phosphor screen by the pair of the side beams are deviated from each other in the vertical direction.
Japanese Patent Disclosure No. 7-21938 teaches that, if three electron beams are to be converged, a pair of the side beams are caused to have components opposite to each other in the direction of the X-axis. It is also taught that, where an external magnetic field running in an axial direction of the color cathode ray tube, i.e., Z-axis, is applied to the electron beams under the particular state noted above, the images displayed on the phosphor screen by the side beams are deviated from each other in the vertical direction because of the Lorentz force.
In order to prevent the images displayed on the phosphor screen by the side beams from being deviated from each other, a pair of magnetic bodies 9 serving to shield the external magnetic field running in the axial direction of the tube are arranged as shown in FIG. 2. As shown in the drawing, these magnetic bodies 9 are arranged to extend in the axial direction of the tube on both outer surfaces of the neck 5.
In general, the magnetic body 9 is fixed to the inner surface of a cylindrical holder H in the convergence magnet 8 in a manner to extend in the Z-axis direction as shown in FIG. 2 in order to decrease the number of mounting steps of the magnetic body 9 and to improve the mounting accuracy.
On the other hand, the 6-pole magnet plate 10 has a total of 6 N- and S-poles alternately arranged equidistantly and generates a magnetic field as shown in FIG. 3. The particular distribution of the magnetic field permits force of the same direction to be exerted on the electron beams on both sides so as to change the orbits of the side beams. Also, the magnet plate 10 is designed such that the magnetic field intensity is off-set so as to become substantially zero on the central axis of the color cathode ray tube, i.e., on the orbit of the central beam, with the result that force for changing the orbit does not act on the central beam.
It should be noted that, if the convergence magnet forming a static magnetic field for correcting the orbits of the three electron beams and the magnetic bodies for shielding the external magnetic field are arranged in the neck portion having a limited space, it is unavoidable for the band-like magnetic body and the ring-like magnet plate to cross each other in the neck portion. Where the magnetic body and the magnet plate are arranged close to each other, the magnetic body is magnetized by the action of the magnet plate, particularly, the magnetic poles of the 6-pole magnet plate, giving rise a serious problems as described below.
Specifically, FIGS. 4A and 4B collectively show the distribution of the magnetic field formed by the 6-pole magnet plate and the magnetization of the magnetic body, covering the case where the orbits of the two side beams are corrected vertically upward, i.e., in a positive direction of the Y-axis. In this case, an N-pole and an S-pole of the 6-pole magnetic plate 10 are positioned to face each other, as apparent from FIG. 4A. It is seen that the magnetic bodies 9a and 9b arranged on the X-axis in a manner to face each other are positioned close to the N-pole N1 and the S-pole S2 of the 6-pole magnetic plate 10, respectively. FIG. 4B shows in a magnified fashion the positional relationship between the magnetic body 9a and the 6-pole magnet plate 10.
Since the magnetic body 9a is positioned close to the N-pole N1 of the magnet plate 10 as described above, that region of the magnetic body 9a which is positioned closest to the N-pole of the magnet plate 10 is magnetized to form an S-pole, i.e., the opposite polarity, as shown in FIG. 4B. This is also the case with the magnetic body 9b positioned close to the S-pole S2 of the 6-pole magnetic plate 10. The S-pole formation in, for example, the magnetic body 9a noted above causes the entire magnetic body 9a to be magnetized such that N-poles are formed at the front and rear end portions.
In short, an S-pole is formed in that surface of the magnetic body 9a which faces the N-pole N1 of the magnet plate 10. Also, N-poles are formed at the front and rear edges of the magnetic body 9a. Likewise, an N-pole is formed in that surface of the magnetic body 9b which faces the S-pole S2 of the magnet plate 10. Also, S-poles are formed at the front and rear edges of the magnetic body 9b. As a result, a magnetic field running in the direction of the X-axis from the magnetic body 9a to the magnetic body 9b is formed at the rear end portions of the magnetic bodies 9a, 9b. The particular magnetic field exerts an upward force to the electron beams passing through the rear end portions of the magnetic bodies.
It should also be noted that a magnetic flux generated from the N-pole N1 of the magnet plate 10 runs partly through the S-pole formed in the magnetic body 9a toward the N-poles at both end portions of the magnetic body 9a. Naturally, the magnetic flux component running from the N-pole N1 toward the S-pole S2 of the magnet plate 10 is weakened. As described previously, when the magnetic bodies 9a, 9b are not disposed, the 6-pole magnet plate 10 is designed such that the magnetic fluxes generated from the N-poles N1, N2, N3 and running toward the S-poles S1, S2, S3 are canceled each other in the central portion of the magnet plate 10. As a result, the magnetic field intensity is substantially zero in the central beam passing point within the magnet plate 10. Where the magnetic bodies 9a, 9b are disposed as shown in FIG. 4A, 4B, however, the magnetic field generated from the N-pole N1 and running toward the S-pole S2 is weakened as described above. As a result, the magnetic field generated from the N-poles N2 and N3 and running toward the S-poles S1, S3 is relatively intensified. It follows that the central electron beam passing point within the magnet plate 10 is in a magnetic field running in the positive direction of the X-axis, i.e., toward the N-pole N1 of the magnet plate 10. On the other hand, the side beam passing points within the magnet plate 10 are in a magnetic field running in the negative direction of the X-axis, as apparent from the drawing. It follows that the central beam and the side beams are put in magnetic fields running in opposite directions within the magnet plate 10.
As described above, a magnetic field running in the positive direction of the X-axis is exerted on the central beam emitted from a central cathode 16 before the central beam runs to reach the deflection device 6. On the other hand, a magnetic filed running in the negative direction of the X-axis is exerted on the side beams emitted from side cathodes 16 before the side beams run to reach the defection device 6. It follows that the side beams within the magnet plate 10 receive an upward force, i.e., positive direction of the Y-axis, with the central beam within the magnet plate 10 receiving a downward force.
Suppose the 6-pole magnet plate 10 is designed such that, when the magnetic bodies 9a, 9b are not used, a magnetic field is not exerted on the central beam and, thus, the central beam is not shifted, within the magnet plate 10 and that each of the side beams is upwardly shifted by 1.3 mm within the magnet plate 10 because of the interaction between the electron beam and the magnetic field. In this case, when the magnetic bodies 9a, 9b are mounted, each of the side beams is shifted upward by 0.5 mm, and the central beam is downwardly shifted by 0.8 mm.
Clearly, the operability of the magnet plate is poor. In addition, since the central beam is shifted in the step of correcting the orbit of the beam by the 6-pole magnet plate 10 after the landing adjustment performed by the two-pole magnet, the central beam must be further controlled again by the two-pole magnet. It follows that the beam control operation is low in efficiency.
As described above, the conventional color cathode ray tube having magnetic bodies mounted therein gives rise to the problem that, when the orbits of the electron beams are corrected in a vertical direction, the shifting amount of the side beam is decreased and, at the same time, the central beam is shifted in an opposite direction.